Carrier boards made of wood or wood-based materials are widely used in furniture making and the building and construction industry. Depending on the dimensions of the wood parts, distinctions are drawn between layered materials, chip materials and fibrous materials. The mechanical properties can be adjusted to great extents in a targeted manner by varying material and process parameters. Dominant influencing parameters are the dimensions of the wood parts, the bulk density of the material, the binder content and the structure of the material perpendicularly to the board plane and in the board plane. For example, in the case of light insulating boards, the flexural strength thereof may be just 1 N/mm2, yet materials with a flexural strength of much greater than 100 N/mm2 can also be produced from the same type of wood having laminated wood parts as the structural elements. In addition to the mechanical properties, in particular the hygric properties also determine the usage possibilities. Materials having binders which are hydrolytically less stable are not suitable for exterior use and in construction elements in which there is a risk of high levels of dampness. For load bearing usages, there are strict regulations regarding the moisture resistance of the materials. The wood components, however, cannot be protected against hygric swelling and shrinkage even by using moisture-resistant binders. The stresses associated with the hygric length changes can be greater than the maximum adhesive tensions between the binder component and the wood component. These relationships result in the delaminations that can often be seen in wood-based materials that are used outside, e.g. as cladding. To prevent these undesirable weathering effects, the materials are coated, varnish typically being applied thereto. A coat of varnish, however, only has a temporary effect. When the varnish layer thickness is very low, unavoidable hygric length changes lead to small cracks, through which water can infiltrate the material. This leads to the varnish separating from the material and to increasing weather damage to the wood-based-material boards.
Although longer lasting coatings made of melamine papers or phenol papers are possible, these are only rarely used, since they cover wood structure. Moreover, these coatings are very brittle. Mechanical loads thus also easily lead to cracks, leading to the same consequences as known with varnish coatings. Crack formation in the region of fastening elements have to be dealt with particularly frequently. With screw heads or nail heads and with nuts, star-shaped cracks, which are initially invisible but become clearly visible in the event of water ingress, form as a result of excessive local stresses. In addition to an aesthetic defect, this also constitutes a technical defect in the longer term.
Some wood-based materials, such as oriented strand board (OSB), consist of relatively large laminated wood parts as functional structure elements (FSE). Owing to the plywood-type design perpendicularly to the board plane and the high mechanical properties, these materials are a preferred construction material. Materials for use in construction have to comply with the legal requirements. For example, the requirements of OSB are controlled by standards, four material classes being defined in EN 300. As the numbers get higher from OSB 1 to OSB 4, the requirements increase, in particular on moisture resistance. OSB 3 is in particular demand and is produced on an industrial scale. The production of OSB 1 is insignificant; OSB 2 and OSB 4 together make up just 20 to 30% of the production volume.
In all wood-based materials, the bulk density is one of the decisive factors in terms of the hygric and mechanical properties. As the density increases, so too do the resilient characteristics (elastic modulus) and the mechanical properties (flexural strength, tensile strength). Also, the amount and type of adhesive are reflected in the mechanical properties. Even with a correspondingly high bulk density, insufficient adhesion manifests itself in low transverse tensile strengths. In OSB, the required amounts of adhesive are approximately the same as in typical chipboards, despite the significantly lower particle surface. A large number of tests further prove the high dependency of flexural strength and bending elastic modulus on the strand length, the ratio of the strand length to the strand thickness (lengthwise degree of slenderness) in fact being more important than the strand length. Significant increases in strength can be achieved up to degrees of slenderness of 200. Therefore, the focus is not on strand thickness during production of OSB either, but rather on the degree of slenderness. Often, the materials contain strands having a thickness of significantly greater than 0.5 mm. Thicker strands cannot sufficiently plasticise even during hot-pressing, and so the surface contact between the individual strands or veneer strips is not comparable to that of plywood. Moreover, the adhesive is not applied to the strands across the whole surface, but rather in a punctiform manner. If these materials are exposed to direct weathering, the result is clearly visible delaminations in the cover layer within a time period that is relatively short for construction materials. Strands can be separated from the surface of the board without using force or can be pulled off using slight peeling stress. Delaminations can occur even in the absence of weathering if the amount of adhesive is insufficient. Since the distribution of adhesive follows statistical laws, relatively high amounts of adhesive are used to largely prevent such delaminations.
The comparably high requirements for adhesive are also due to the pore structure which is inevitably produced in the case of relatively thick and thus insufficiently plasticised strands or veneer strips. In the event of tensile stress and unfavourable pore structure, the adhesive joints are subject to peeling—a very disadvantageous type of stress for a bonded joint.
Pore space and board bulk density are in approximately linear correlation up to a bulk density of approximately 10% over the bulk density of the wood. Above this range, the inter-particle pore space does not decrease by the same amount as the board bulk density increases—an effect which is influenced, inter alia, by the strand geometry, strand thickness, wood type, plasticisation during hot-pressing, and strand moisture prior to the hot-pressing. For commercial and technical reasons (practical board bulk density, strand thickness variations), in industrial conditions a certain pore space will always remain in the wood-based materials consisting of strands and veneer strips.
For specific applications, materials having a high elastic modulus are also required in furniture making. Materials having strands and veneer strips would, for example, be very suitable materials for furniture. For visual and usage reasons, furniture materials need to be coated. The most widely used coatings are melamine papers, films and laminates, which are applied by means of a pressing operation with or without the application of adhesive. Although it is possible to apply these coating materials to the material surfaces, as a result of the coating the pores that are also present in the surface begin to show, or the coating is pushed into the pores. Previous approaches to solving this problem by applying several coating layers, by priming or by laminates were successful in technical terms but were not economical. There is thus the need for a furniture material which can be coated and has a high elastic modulus.
If the delaminations could be prevented, the strand and veneer strip materials would also be well suited as a construction material in regions exposed to high amounts of moisture. Protection against delaminations could be achieved by means of coatings. In this way, press coatings such as melamine papers and phenol papers can prevent water ingress effectively. The pore space near the surface, however, is not completely filled by these coatings, but rather is merely covered in a sheet-like manner. These regions constitute weak points to mechanical damage, and therefore only the very expensive multi-layer press coatings or laminates ensure long-lasting protection. Aside from a few usage areas, for example formwork boards, press coatings on strand and veneer strip materials could not become widely accepted.
In addition to the less favourable press coatings (melamine papers, films), varnishes are also conceivable as coatings. Since deep surface pores are also disadvantageous for varnish coatings and additionally it is not possible to produce a covering coating using the typical application amounts owing to the pore structure, ingress of water by capillary action and sorption cannot be avoided with a varnish coating either.
Therefore, for applications in both furniture and construction, there is a need for effective coating materials made of strands and veneer strips which fill pores, repel water, are long-lasting and are less susceptible to cracks.
In addition, wood polymer composites (WPC) are known. These WPCs based on polyolefins and other thermoplastically workable plastics materials are becoming increasingly established in the German and European markets. They are largely used outside. Typical uses are decking coverings and façade cladding. The aim is for their potential uses to be expanded, though this is greatly restricted at the moment by a number of features, including the low surface energy thereof, which makes bonding or coating significantly more difficult. Components made of WPC are described, for example, in DE 10 2006 027 982 B3 and DE 20 2009 448 U1, with DE 20 2009 448 U1 describing an extrusion method and DE 10 2006 027 982 B3 describing an injection moulding method for producing the respective components.
The generic document, EP 1 847 385 A1, proposes a multi-layer board consisting of a core and cover layers arranged on the top side and bottom side of the core. In particular, it is proposed that the cover layers consist of WPC and the core is an OSB board. A method is disclosed for the production of a multi-layer board. First, a layer of WPC granular material is strewed on, onto which a layer of OSB strands is strewed. A layer of WPC granular material is strewed onto this layer. The three layers are then jointly pressed together under the effect of heat and pressure.
However, this method has been shown to be unworkable in practice because the granular material strewed onto the upper cover layer runs into the loose mat and is thus not fixed onto the strands, does not bring about complete covering, and leads to an asymmetrical board structure. An asymmetrical board structure causes the boards to become warped, as a result of which they become unusable.